The present invention relates in general to an electrode for gas discharge devices, and more particularly to hollow cathodes for use in high-pressure, high-power discharge devices
In general a low-pressure (i.e., lower than one torr) hollow cathode discharge, is a spectroscopic light source of relatively high emission efficiency, low power consumption and small line width through Doppler broadening. The hollow cathode could assume several designs, including a hollow cylindrical cathode or a double (i.e two-plate) cathode. The characteristics of a hollow cathode discharge device are described in an article entitled "New Hollow Cathode Glow Discharge" by A. D. White, in the Journal of Applied Sciences, volume 30, number 5, May 1959 issue.
In a hollow cathode discharge device, the region that is adjacent to the cathode surface is referred to as the "cathode fall region", "cathode dark space" or "cathode hollow space". The voltage drop across this cathode fall region is referred to as the "cathode-fall voltage", and the thickness of this region is referred to as the "cathode-fall thickness". The cathode-fall voltage varies proportionally to the discharge-current density (i.e the electrical current per unit electrode surface area), while the cathode-fall thickness varies inversely with respect to the discharge current density.
In high-power gas discharge devices, such as lasers, gaseous switches, flash lamps and the like, the current density at the electrode is usually very high because of the high discharge current requirement. A high current density would require a high cathode-fall voltage, and therefore, a smaller cathode-fall thickness at the cathode. As a result of a large cathode-fall voltage drop across the small cathode-fall thickness, the electrode impedance is high, which causes excessive energy to be "deposited" or dissipated as heat at the cathode. This large energy deposition can initiate "thermal run-away effect".
Such thermal run-away effect results in "discharge instability" at the cathode. Additionally, when higher pressure gases are used in the gas discharge device, the cathode impedance increases further, and the thermal run-away effect worsens.
In a stable discharge, the discharge at the electrode follows a diffused glow pattern. On the other hand, when discharge instability occurs, the discharge pattern at the cathode appears as several confined intense "streamers", which move along the electrode surface in an uncontrollable pattern. Such intense streamers will degrade the discharge performance of the cathode, and will shorten its effective durability.
Additionally, sputtering debris is produced at the cathode, and is caused to drift away from the cathode, toward the anode and the gas or plasma chamber. The debris contaminates the discharge chamber, and further reduces the efficiency of the discharge device. When the efficiency reaches, or drops below an unacceptable level, the discharge device is either replaced or refurbished.
Furthermore, conventional hollow cathodes have relatively small cathode areas, so as to effectively maintain what is referred to as "hollow cathode effect". The hollow cathode effect occurs when the primary electrons, which are initially produced at the cathode, enter the fall region of one cathode plate, and after retardation, are "repelled" and accelerated to the fall region of the other cathode plate.
The electrons are repeatedly bounced back and forth between the two cathode plates, traversing the hollow space therebetween, and generating additional secondary electrons. Such oscillatory motion of the electrons continues until the electrons move away from the hollow space, toward the anode, due to the cathode-anode field.
Wherefore, the hollow cathode effect has the following two characteristic features:
1. The photo-electric (i.e, photon induced) electron emission from the cathode is more efficient because more UV photons from the cathode glow are captured by the hollow cathode. The two plates of the cathode confine these UV photons within the narrow hollow space. PA1 2. The oscillatory travel of the electrons within the hollow space increases the length of the travel path of the electrons, which greatly enhances the electron multiplication through collisional ionization with neutral gas atoms.
As a result, a hollow cathode can achieve higher current density at a lower cathode fall voltage, because of a more efficient free-electron generation. However, the size of conventional hollow cathodes is relatively small because the size or thickness of the hollow space must be kept small enough, in order to maintain an efficient hollow cathode effect.
For example, the multiplication result of the cathode-fall thickness and the gas pressure is normally kept within the 0.1 cm-torr range, for an efficient hollow cathode discharge. Consequently, the use of the hollow cathode discharge devices in high-pressure, high-power applications is severely limited, due to the size limitation of the cathode in particular, and the discharge device in general.
Therefore, it would be desirable to have a new hollow cathode which is efficiently usable in high-pressure, high-power gas discharge devices.